1. Field of the Invention
This invention relates to improved magnetic apparatus for precisely positioning an X-Y movable stage with respect to a fixed base and, more particularly, to such a stage suitable for use in a wafer-stepper employed in the photolithographic manufacture of monolithic integrated circuits.
2. Description of the Prior Art
In the past, stepper motors have been used for the purpose of positioning the X-Y stage of a wafer-stepper with six degrees of freedom. Recently, however, prior-art magnetic positioning apparatus has been developed for this purpose. This magnetic positioning apparatus provides substantially higher positioning resolution and other advantages over stepper motors. These other advantages include:
(1) the monolithic nature of such magnetic positioning apparatus allows direct coupling between the metrology system, the stage, and the wafer or substrate; PA1 (2) the direct electromagnetic coupling in all 6 axes eliminates the need for precision robbing, slow mechanisms such as focus actuators and coarse theta adjustment mechanisms; PA1 (3) the design of such magnetic positioning apparatus lends itself to a high bandwidth, servo controlled, positioning system that settles much faster (because of the higher bandwidth) than other types of positioning stage; and PA1 (4) such magnetic positioning apparatus is equally applicable to step-and-repeat or step-and-scan types of operation and does not exhibit cogging that inhibits travel at constant velocity with Sawyer motor based stages.
A first example of a prior-art magnetic positioning apparatus suitable for use in a wafer-stepper is disclosed in U.S. Pat. No. 5,196,745, issued Mar. 23, 1993. The magnetic positioning apparatus disclosed in U.S. Pat. No. 5,196,745 employs a plurality of stage-attached permanent magnetic arrays, each of which comprises a series of adjacent oppositely-poled permanent magnets (i.e., magnetic fields of each pair of adjacent magnets are rotated 180.degree. with respect to one another) that cooperate with stationary electromagnetic coil arrays in the horizontal (X, Y) plane, to produce the proper lateral forces to provide desired motion of the X-Y stage in X and/or Y directions. Additional electromagnets provide controllable forces for levitating the stage-attached permanent magnetic arrays' forces in the vertical (Z) direction with respect to the stationary electromagnetic coil arrays to maintain a desired air gap therebetween and controllable couples for providing small angular rotations of the stage-attached permanent magnetic arrays about the X, Y and/or Z axes.
Known in the art is a so-called Halbach magnetic array, which comprises a series of permanent magnets in which the respective magnets are poled so that the magnetic fields of each pair of adjacent magnets are rotated 90.degree. with respect to one another
A second example of prior-art magnetic positioning apparatus for use in a wafer-stepper is shown in FIGS. 1 and 2 and is described in detail below. However, briefly, this second example comprises four spatially-separated Halbach magnetic arrays extending from each of the four corners of an X-Y stage, with each of the Halbach magnetic arrays cooperating with a stationary electromagnetic coil array in the horizontal (X, Y) plane, thereby achieving (1) controllable lateral forces in the X and Y horizontal directions and (2) controllable levitating forces in the vertical Z direction which are also capable of creating controllable couples for providing small angular rotations about the X, Y and/or Z axes. Thus, this second example of prior-art magnetic positioning apparatus differs from the above-described first example of prior-art magnetic positioning apparatus in that it does away with the need for the aforesaid additional electromagnets for providing both controllable levitating forces in the vertical Z and controllable couples for providing small angular rotations about the X, Y and/or Z axes.
Currently, wafers as large as 300 millimeters (mm.) in diameter need to be processed. Thus, the overall horizontal area of the second example of the prior-art magnetic positioning apparatus for use in a wafer-stepper for processing 300 mm is very large (significantly greater than 4.times.4=16 square feet). The problem caused by this very large horizontal area is that it makes it very hard to adequately support the photolithographic projection optics, while also complicating the layout of the wafer-stepper.